pressure vessels intro

8/13/2019 Pressure Vessels Intro

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Pressure

Vessels


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A Pressure Vessel is acontainer for fluids underpressure between 15 psig

to 3000 psig

ASME Boilers and Pressurevessel Code Section VIIIDivision I sets rules for thedesign, fabrication, andinspection of pressurevessels


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Main PV Components andConfigurations


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The shell is the primary component thatcontains the pressure. Curved shape

Vessel may be cylindrical, spherical, orconical

The Vessel is always closed by heads

The components are typically weldedtogether


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Multiple diameters, thicknesses ormaterials are possible

The Saddle supports used for horizontaldrums

(1) Spreads load over shell;(2) One support is fixed, the other

slides


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Most heads are curved shape forstrength, thinness, and economy

Semi-elliptical shape is most common

head shape


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Nozzles used for:

> Piping systems

> Instrument Connection

> Manways

> Attaching other equipments

Ends are typically flanged, may be welded


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Spherical storage vessels are typicallysupported on legs

Cross- bracing is typically used toabsorb wind and earthquake loads


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Vessel size limits for lug support:

> 1 to 10 ft diameter

> 2:1 to 5:1 height to diameter ratio

Lugs are bolted to horizontal structure


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MATERIAL SELECTION

Material Selection Factors:

> Strength

> Corrosion Resistance

> Resistance to Hydrogen Attack

> Fracture Toughness

> Fabricability


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MATERIAL SELECTION FACTORS

Strength matls ability to withstandimposed loading

Determines reqd component thickness

Overall strength determined by:

> Yield Strength> Ultimate Tensile Strength> Creep Strength

> Rupture Strength


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Corrosion Resistance Deterioration ofmetal by chemical action

MOST IMPORTANT factor to consider

Corrosion allowance supplies additional

thickness

Alloying elements provide additional

resistance to corrosion


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Resistance to Hydrogen Attack

At 300 400 F, monoatomic hydrogen formsmolecular hydrogen in voids

Pressure buildup can cause steel to crack

Above 600 F, hydrogen attack causes irreparabledamage through component thickness

Increased alloy content (i.e. Cr) increase H 2

attack resistance


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Brittle Fracture and Fracture Toughness

Fracture toughness ability of matl towithstand condns that cause brittlefracture

Brittle fracture

> Typically at low temperature > Can occur below design pressure

> No yielding before complete failure


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Brittle Fracture and Fracture ToughnessConditions for Brittle Fracture to Occur:

> High enough stress for crack initiation

and growth> Low enough matl fracture toughness at

temperature> Critical size defect to act as stress

concentration

Brittle Fracture occurs w/o warning and iscatastrophic


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Fracture Toughness Varies with:> Temperature> Type and Chemistry of steel> Manufacturing and Fabrication

processes> Arc strikes, esp. if over repaired area> Stress raisers or scratches in cold formed

thick plate


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Brittle Fracture and Fracture ToughnessSimplified ASME Evaluation Approach

Material specification classified into MaterialGroups A to D

Impact test exemption curvesFor each material groupAcceptable MDMT vs. thickness where

impact testing not required> If combination of Material Group and

thickness not exempt, then must impact

test at CET


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Additional ASME Code Impact Test Requirement s

-Required for welded construction over 4 in. thick,or non-welded construction over 6in. thick,if MDMT < 120 F

-Not Required for Flanges if temperature -20 F

-MDMT reduction if calculated stress < allowable

stress


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Fabricability

Ease of Construction

Any required special fabrication practices

Material must be weldable

pressure vessels intro

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